Exposure to light is the key mechanism that enables a proper synchronization of the body clock with the solar day cycle. Timing, duration, intensity and spectral composition of light exposure all have impact on the so-called entrainment of a person to a 24-hour circadian rhythm. It has been shown that restorative sleep can only occur in synchronicity with the body clock. For certain people who have a phase shift of their internal body clock relative to the social schedules around them, exposure to bright light at well-defined times can be used to shift their body clock forward or backwards to better align it with their social needs. Also for the treatment of seasonal affective disorder, timed and regular exposure to bright light is an effective means.
The assessment of light levels to which a person is exposed over the course of multiple days or even weeks is an important instrument for the diagnosis of mood and sleep disorders. Existing actigraphy products measure the activity and light exposure with a device that is used and worn similar to a wrist watch.
Such a wrist-worn device is shown, for example, in US 2008/0319354 A1, showing a system and method for monitoring information related to sleep. The wrist-worn device shown in this document comprises an illumination sensor to provide information related to the intensity of ambient illumination of the user. The signal of the sensor can be further processed by suitable electronic computing means.
The common sensor modules are usually mounted such that the axis of main sensitivity is orthogonal to the display surface of the operation module, i.e., orthogonal to the outer hand surface of the user. At the same time, the axis of main sensitivity of the sensor is completely independent from the direction of gaze of the user, with the consequence that there is no good correlation between the light levels being measured and those light levels present at the user's eyes which would be optimal for the estimation of the visual and non-visual effects caused by light exposure such as e.g., shifting human circadian phase. Furthermore, generally the main axis of sensitivity is different from the direction from which the main portion of ambient light is received. In most environments light is not being uniformly received from all directions, but the main exposure is caused by a small number of light sources (the sun, lighting on the ceiling, etc.). It follows from the above that using wrist-worn devices as described above not always leads to satisfactory results in sensing the ambient lighting and estimating its influence on the visual and non-visual effects as e.g., shifting human circadian phase.